The invention relates to a transmission unit for a muscle-powered vehicle.
In recent decades, the basic principles of gearshift mechanisms for bicycles in particular have not changed substantially. The gearshift mechanism is primarily located on the rear axle, wherein two systems have substantially become popular, namely derailleur gearshift mechanisms on the one hand and internal-gear hubs on the other. In both systems a chain is used to transmit the drive force to the rear axle of the bicycle.
The term derailleur gearshift mechanism is generally used to describe a pack of sprockets mounted on the rear axle comprising up to ten sprockets, between which it is possible to shift back and forth by means of a shift mechanism which is mounted on the frame and serves to guide the chain. In order to widen the transmission range, most bicycles are equipped with an additional gearshift mechanism at the chain wheel of the bottom bracket. For this purpose, up to three chain wheels are attached on one side of the foot pedals, between which it is possible to shift back and forth by means of a front derailleur mounted on the frame. This type of derailleur gearshift mechanism offers up to 30 gears and an overall transmission ratio of up to approximately 600%. However, due to the nature of the design and depending on the layout, many of the gears are redundant on derailleur gearshift mechanisms. For example, a standard commercially available derailleur gearshift mechanism with 27 gears only has 15 non-redundant gears.
The components of a derailleur gearshift mechanism are mounted externally on the frame of the bicycle and are hence directly exposed to environmental influences, such as water and dirt. As a result, the high efficiency of a derailleur gearshift mechanism is reduced particularly quickly, which means that a large proportion of the expended muscular energy is lost through friction. Furthermore, derailleur gearshift mechanisms require very precise adjustment in order to ensure accurate performance. Due to the outward-lying design, the components are susceptible to damage caused by falls or contact with stones or branches. As the chain runs at an—in some cases large—angle, and due to the frequent derailing of the chain between sprockets, the chain and the sprockets are especially subject to high wear and require regular replacement. The known gearshift mechanisms for bicycles thus have the disadvantage that, in order to ensure perfect operation and a high level of efficiency, the level of maintenance required for a derailleur gearshift mechanism is correspondingly high and cost-intensive.
By contrast, the term internal-gear hub is used to describe a gearing mechanism which is integrated in the hub housing of the rear axle. In the case of most internal-gear hubs, the gearing mechanism comprises a plurality of planetary gear stages between which it is possible to shift back and forth. In comparison to derailleur gearshift mechanisms, an internal-gear hub has no outward-lying gearshift components such as shift unit, sprocket pack, a plurality of chain wheels or derailleurs. Consequently, internal-gear hubs are more low-maintenance than derailleur gearshift mechanisms, but they offer lower overall transmission ratios. An internal-gear hub, such as is disclosed for example in DE 197 20 794 A1, currently has up to 14 gears and an overall transmission ratio of up to 524%. However, due to the arrangement of the internal-gear hub on the hub of the rear axle, on the one hand the weight of the rotating mass is increased, while on the other hand in the case of bicycles with rear suspension the unsprung mass seen in relation to the overall weight is also increased. Furthermore, the centre of gravity of the bicycle is also shifted towards the rear axle. These factors have a disadvantageous effect on the riding characteristics of bicycles, particularly in the case of mountain bikes with rear suspension.
Bicycles having a gearing mechanism which is positioned in close proximity to the bottom bracket (pedal bearing) and is integrated in the frame represent a recent speciality. Such a gearing mechanism is described, for example, in DE 10 2004 045 364 A1 or DE 103 39 207 A1. In order to realise such a gearing mechanism, a plurality of intermediate shafts having switchable gear wheels is provided in a gear housing. The bottom bracket either directly forms part of the gearing mechanism, or a rotary motion is initially transmitted from the bottom bracket via a drive mechanism onto the gearing mechanism. Starting from an output shaft of the gearing mechanism, the force is then, in the same manner as in the previously mentioned gearshift mechanisms, transmitted by means of a chain drive to the rear wheel. One particularly advantageous feature of these gearshift mechanisms is the fact that the mass of the gearing mechanism tends to lie in the proximity of the centre point of the bicycle, so that the centre of gravity is hence influenced in a positive manner. Furthermore, the components of the gearing mechanism are protected inside the gear housing against the effects of weathering. These gearshift mechanisms currently offer up to 15 gears and an overall transmission ratio of up to 646%. However, the disadvantages of this type of gearing mechanism include their large space requirements and high weight. In order to minimise drive influences, on many bicycles with rear suspension the instantaneous centre of rotation of the axis of rotation of the rear axle suspension lies in close proximity to the bottom bracket. The drive influences are minimised if the axis of rotation lies exactly in the point at which the chain lifts off from the chain wheel. However, the output axes of the gearing mechanisms described here do not lie in close proximity to the bottom bracket, and are instead positioned at a noticeable distance from the latter, so that a completely new layout of the rear suspension needs to be designed in order to minimise the drive influences in turn.